Biomarkers are increasingly important to diagnose and test treatments of neurodegenerative diseases such as Parkinson disease (PD). We compared neuroimaging, neurochemical, and olfactory potential biomarkers to detect central dopamine (DA) deficiency and distinguish PD from multiple system atrophy (MSA). In large groups of patients with PD or MSA and in control subjects, radioactivity concentrations in the putamen (PUT), caudate (CAU), occipital cortex (OCC), and substantia nigra (SN) were measured 2 hours after 6-(18F)fluorodopa injection, septal myocardial radioactivity was measured 8 minutes after 6-(18F)fluorodopamine injection, CSF and plasma catechols were assayed, and olfactory function was tested using the University of Pennsylvania Smell Identification Test (UPSIT). Receiver operating characteristic curves were constructed, showing test sensitivities at given specificities. PUT:OCC, CAU:OCC, and SN:OCC ratios of 6-(18F)fluorodopa-derived radioactivity were similarly low in PD and MSA, as were CSF dihydroxyphenylacetic acid (DOPAC) and DOPA concentrations. PUT:SN and PUT:CAU ratios were lower in PD than in MSA. CSF DOPAC correlated positively with PUT:OCC ratios. Myocardial 6-(18F)fluorodopamine-derived radioactivity distinguished PD from MSA (83% sensitivity at 80% specificity, 100% sensitivity among patients with neurogenic orthostatic hypotension). Only PD patients were anosmic; only MSA patients had normal olfaction (61% sensitivity at 80% specificity). PD and MSA therefore both feature low PUT:OCC ratios of 6-(18F)fluorodopa-derived radioactivity and low CSF DOPAC and DOPA concentrations, cross-validating the neuroimaging and neurochemical approaches but not distinguishing the diseases. PUT:SN and PUT:CAU ratios of 6-(18F)fluorodopa-derived radioactivity, cardiac 6-(18F)fluorodopamine-derived radioactivity, and olfactory testing separate PD from MSA.[unreadable] [unreadable] Pure autonomic failure (PAF) and PD share several clinical laboratory abnormalities; however, PAF is not associated with parkinsonism. We tested the hypothesis that preservation of nigrostriatal dopaminergic innervation explains the absence of motor dysfunction in PAF. Patients with PAF or PD and control subject had brain 6-(18F)fluorodopa positron emission tomographic scanning and CSF catechol measurements. A patient with PAF and another with PD had rapid postmortem striatal, nigral, and sympathetic ganglion sampling, with assays of catechols and tyrosine hydroxylase (TH) activity. The PAF and PD groups had similarly low mean substantia nigra (SN):occipital (OCC) ratios of 6-(18F)fluorodopa-derived radioactivity and similarly low CSF DOPAC and DOPA levels. Only the PD group, however, had low PUT:OCC, caudate:OCC, or PUT:SN ratios. The PAF and PD cases had similarly low SN tissue concentrations of dopamine (DA) and TH activity, but the PD patient had tenfold lower PUT dopamine and the PAF patient 15-fold lower myocardial norepinephrine (NE) concentrations. PAF and PD therefore entail similarly severe nigral and overall central dopaminergic denervation. There is more severe loss of striatal dopaminergic terminals in PD than in PAF and more severe loss of sympathetic noradrenergic terminals in PAF than in PD. These differences explain the distinctive clinical manifestations of the two Lewy body diseases. Parkinsonism appears to reflect striatal dopamine deficiency rather than loss of nigral dopaminergic neurons per se.[unreadable] [unreadable] We are pursuing the above findings, by a new clinical Protocol, entitled, Biomarkers of Risk of Parkinson Disease. This Protocol is to test whether individuals with putative risk factors for PD have abnormal values for biomarkers of central or peripheral catecholaminergic innervation and whether at-risk individuals with positive biomarkers develop PD within up to 7.5 years of follow-up. We are also awaiting results of a large collaborative study of CSF proteomics in patients with PD (with or without neurogenic orthostatic hypotension), MSA, or PAF. Under a Material Transfer Agreement and Amendment to one of our clinical research Protocols, we plan to assay tissue catechols and TH activity in skin biopsy specimens from patients with synucleinopathies. Finally, using a new LC/MS/MS system, we hope to develop novel, high-throughout, high-sensitivity assay methods for catecholamines and their metabolites in plasma and CSF, to identify other biomarkers of central or peripheral catecholamine deficiency in synucleinopathies.